1. Field of the Invention
The invention relates to the field of quantum electronics and to repetitively pulsed gas lasers having a transverse self-maintained discharge and UV pre-ionization.
2. Discussion of the Related Art
The energy stability of a gas discharge laser such as an excimer laser is strongly influenced by the strength and uniformity of the preionization of the laser gas within the discharge volume. The xe2x80x9cpreionizationxe2x80x9d of the laser gas corresponds to the initial electron concentration in the discharge volume at the initial stage of the discharge sequence. There have been developed several types of preionization devices and methods for generating short wavelength UV radiation that interacts with the laser gas in the discharge volume.
One type of UV preionization system that is used in gas lasers typically provides self-maintained discharges from a number of spark sources disposed near one or both electrodes on one or both sides of them (see, e.g., V. Yu. Baranov, V. M. Borisov and Yu. Yu. Stepanov, xe2x80x9cElektrorazryadnye eksimernye lazery na galogenidakh inertnykh gazovxe2x80x9d (Electric discharge excimer lasers based on the halides of inert gasesxe2x80x9d), Energoatomizdat, 1988; and see U.S. Pat. Nos. 4,105,952, 4,980,894, 4,951,295, 4,797,888, 5,347,532 and 4,287,483, which are hereby incorporated by reference). FIG. 1 illustrates an electric discharge circuit (e.g., capacitor discharging circuit) of a gas laser having spark UV pre-ionization, comprising a high-voltage electrode 1, a grounded electrode 2, a storage capacitor 3 and a peaking capacitance 4, a commutator (thyratron) 5, and one to four rows of UV-pre-ionization spark sources 6. In such a circuit, the number of spark sources in each row is discrete and their maximum number is typically determined by the number of peaking capacitors used top provide the peaking capacitance 4.
The spark UV pre-ionizer operates as follows. The storage capacitor 3 is charged up from the high-voltage supply source to the voltage U0, and after the thyratron has fired, the peaking capacitors are discharged via the spark gap 6. The UV pre-ionization provides a substantial concentration of starter electrons for producing a volume discharge in the interelectrode gap between the electrodes 1 and 2. When the laser is operating in repetitively pulsed mode, the induced gas flow ejects, during the time between the pulses, a plug of heated gas, formed from the main discharge and the spark sources of the preceding pulse and containing products of the plasma-chemical reaction, from the main discharge zone.
Disadvantages of UV-pre-ionization spark systems include the following. Since the number of spark gaps is discrete and generally limited by the finite dimensions of the peaking capacitors, the distribution of the concentration of starter electrons in the main discharge zone will be modulated over the length of the electrodes, thereby reducing the active volume of the laser and, consequently, its output characteristics. This is particularly undesirable in short-wave excimer lasers (e.g., KrF and ArF lasers) and molecular fluorine lasers (i.e., F2 lasers), in which, due to strong photoabsorption of the pre-ionizing radiation, spark sources are typically disposed as near as possible to the main discharge zone.
Another significant disadvantage of the UV-pre-ionization spark system is erosion of the spark source electrodes, which, in the first place, severely contaminates the gas volume and, in the second place, is responsible for a relatively short service life of these spark gap electrodes. In addition, the space modulation of the level of pre-ionization results in the same modulation of the main discharge, which contributes to uneven erosion of the main laser electrodes. Yet another disadvantage of this system of pre-ionization is the complexity of varying the energy passing through the pre-ionizer without changing the total capacity of the peaking capacitors. A significant disadvantage of spark pre-ionization is also the fact that the spark sources form plasma reaction products, and fluctuations in the gas flow which are injected by the gas flow into the inter-electrode region when the laser is operating in repetitively pulsed mode at the instant of the succeeding discharge pulses, which limits the maximum pulse repetition frequency.
Among the various preionization techniques is another technique known as sliding surface discharge preionization (see, e.g., German Gebraushmuster DE 295 21 572 and U.S. Pat. Nos. 5,081,638 and 5,875,207, and U.S. patent application Ser. No. 09/532,276, which is assigned to the same assignee as the present application, each patent reference of which is hereby incorporated by reference), and an additional technique known as corona discharge preionization (see U.S. patent application Ser. Nos. 09/247,887 and 09/692,265, which are assigned to the same assignee as the present application, U.S. Pat. No. 5,247,531, and German patents no. DE 3035730, 3313811, 2932781, and 2050490, all of which are hereby incorporated by reference).
The sliding surface discharge, e.g., according to DE 29521572 and U.S. Pat. No. 5,875,207, each mentioned above, is an efficient and promising method for the preionization of the excimer and molecular fluorine laser gas media. It is a type of discharge at the surface of a dielectric medium. The surface discharge provides radiation in the UV and VUV spectral range down to a wavelength xcex=2 nm at a plasma temperature in the discharge of up to 3xc3x97104xc2x0 K. (see also Bagen B., Arbeitsbr. Ins. Plasma Phys., Julisch 1963, pp. 631-34, which is hereby incorporated by reference).
The ""638 patent, mentioned above, and referring specifically to FIGS. 4a and 4b therein, describes a sliding surface discharge preionization arrangement wherein insulating material is positioned between preionization pins to bridge the gap between the pins. The insulating material provides a xe2x80x9ctracking surfacexe2x80x9d for a preionization discharge. An advantage of the arrangement according to the ""638 patent is the minimization of wear on the electrode pins, which is typically a problem with conventional spark gap preionizer arrangements, as mentioned above. The voltage needed to drive a sliding surface discharge is less than that needed for dielectric breakdown of the gas between the pins. An additional advantage is that significant output laser parameters such as energy stability are more stable for excimer and molecular fluorine lasers having sliding surface discharge preionizers than those having spark preionizers, and longer dynamic gas lifetimes for excimer and molecular fluorine lasers are achievable.
It is desired to provide an improved gas discharge laser system, such as an excimer or molecular fluorine laser system, by providing an improved UV pre-ionization laser source which is based on discharge over the surface of a dielectric. It is therefore an object of the invention to provide a self-initiated, efficient, low-current, spatially uniform UV pre-ionization, which provides high laser output parameters and a long service life both of the gas mixture and of the structural components of the electric discharge system relative to those provided by spark UV pre-ionization, and which is advantageous in view of known sliding surface discharge devices such as those set forth at U.S. Pat. Nos. 5,081,638 and 5,875,207, and german Gebrauchmuster DE 295 21 572 U1.
In accordance with this object and in view of above background discussion, an excimer or molecular fluorine laser is provided including a discharge chamber filled with a gas mixture at least including a halogen-containing species and a buffer gas, a pair of elongated main discharge electrodes within the discharge chamber for forming a discharge therebetween, a sliding surface preionization unit within the discharge chamber including an elongated dielectric plate, a discharge circuit for supplying electrical pulses to the main discharge electrodes to energize the gas mixture and a resonator for generating a laser beam. The discharge circuit further includes a peaking capacitance connected between the main discharge electrodes and a preionization capacitance less than the peaking capacitance connected to the sliding surface preionization unit.
A device intended for self-initiated UV pre-ionization of an active volume of an electric discharge repetitively pulsed gas laser is also provided including an electrode system having capacitors connected to it, wherein, in order to improve output characteristics of the laser and the service life of the gas mixture, to reduce the contamination of the gas cavity and to increase the service life of components of the laser electric discharge system, UV pre-ionization of the active volume is effected by UV irradiation from a discharge over a surface of at least one dielectric plate, which discharge is produced during charging of peaking capacitors proceeding from a low-voltage lead end, which plate is disposed near and on one side of a grounded main discharge electrode relative to a discharge region of the laser.
A device intended for self-initiated UV pre-ionization of an active volume of an electric discharge repetitively pulsed gas laser is further provided including an electrode system having capacitors connected to it, wherein, in order to improve output characteristics of the laser and the service life of the gas mixture, to reduce the contamination of the gas cavity and to increase the service life of components of the laser electric discharge system, UV pre-ionization of the active volume is effected by UV irradiation from a discharge over a surface of at least one dielectric plate, which discharge is produced during charging of peaking capacitors proceeding from a low-voltage lead end, wherein, as a result of the discharge over the surface of the dielectric, only a small proportion of the peaking capacitors is charged, while the remainder are connected to the grounded electrode without having the dielectric plate connected therebetween.